Frequency modulated (“FM”) radio frequency (“RF”) pulses have unique characteristics and certain advantages compared to conventional constant frequency, amplitude modulated (“AM”) RF pulses. By modulating both the frequency and amplitude functions independently, the RF pulse width can be decoupled from its RF bandwidth, which is usually dictated by the pulse shape dependent time-bandwidth product, or R-value. This decoupling facilitates even distribution of RF energy with respect to time by sequentially exciting, inverting, or refocusing spin isochromats, allowing broadband excitation without the need of an increase in peak RF power.
FM pulses are widely used in in vivo magnetic resonance spectroscopy (“MRS”) experiments. Often, these experiments are carried out using surface coils to maximize receive sensitivity given the limited availability of concentration for the nuclei of interest. The spatial variation in flip angle due to highly inhomogeneous B1 fields produced by surface coils is overcome by carefully sweeping the frequency.
Called the “adiabatic condition,” this ensures that the magnetization will follow the effective field during adiabatic rapid passage provided that the effective field is swept at a slower rate than the rotation of magnetization about this effective field. Hence, adiabatic pulses enable uniform rotations of magnetization, even with a B1 field profile that spatially varies by an order of magnitude, as long as a certain B1 threshold is met. This characteristic makes adiabatic pulses highly tolerant to extreme B1 variations.
Spatially selective adiabatic pulses are advantageous for certain applications. For example, they can improve the selectivity of a pencil beam excitation T2-prep sequence when replacing the non-selective adiabatic π pulses. Additionally, multidimensional adiabatic pulses can be used to enable reduced field-of-view (“FOV”) image acquisitions with improved time-efficiency, spatial resolution, or both. They can also be used to enhance navigator signals by selectively targeting a specific organ, thereby generating high fidelity signals that specifically originate from the moving organ. Furthermore, their high tolerance to inhomogeneous transmit B1+ makes them attractive for application at high fields, where inhomogeneity is more pronounced.
Thus, there is a desire to provide systems and methods capable of designing multidimensional, spatially-selective adiabatic pulses.